LTC3450

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3450fa

OPERATIO

The LTC3450 is a highly integrated power converter in-

tended for small TFT-LCD display modules. A fixed fre-

quency, synchronous PWM boost regulator generates a

low noise 5.1V, 10mA bias at greater than 90% efficiency

from an input voltage of 1.5V to 4.6V. Three charge pump

converters use the 5.1V output to generate 10V, 15V and

–5V, –10V or –15V at load currents up to 500µA. Each

converter is frequency synchronized to the main 550kHz

(nominal) boost converter. The generated output voltages

are internally sequenced to insure proper initialization of

the LCD panel. A digital shutdown input rapidly discharges

each generated output voltage to provide a near instanta-

neous turn-off of the LCD display.

Boost Converter

The synchronous boost converter utilizes current mode

control and includes internally set control loop and slope

compensation for optimized performance and simple de-

sign. Only three external components are required to

complete the design of the 5.1V, 10mA boost converter.

The high operation frequency produces very low output

ripple and allows the use of small low profile inductors and

tiny external ceramic capacitors. The boost converter also

loading the input power source. Softstart produces a

controlled ramp of the converter input current during

startup, reducing the burden on the input power source.

Very low operating quiescent current and synchronous

operation allow for greater than 90% conversion effi-

ciency.

The MODE input reduces the boost converter operating

frequency by approximately 8x when driven high and

reduces the output power capability of the boost con-

verter. MODE is asserted when the polysilicon TFT-LCD

display is in its extremely low power blank condition. The

boost converter further reduces its quiescent current in

this mode, delivering both lower input (battery) current

drain and low noise operation.

Charge Pumps

The LTC3450 includes three separate charge pump con-

verters which generate 10V, 15V and either –5V, –10V or

–15V. Each output can deliver a maximum of 500µA. The

charge pumps feature fixed frequency, open-loop opera-

tion for high efficiency and lowest noise performance. The

charge pump converters operate at 1/8 the boost con-

verter frequency and include internal charge transfer

switches. Thus, each charge pump requires only two small

external capacitors, one to transfer charge, and one for

filtering. Similar to the boost converter, the charge pumps

operating frequency reduces to approximately 4kHz in

blank mode, maintaining low noise operation but at re-

duced output current capability.

Output Sequencing

Refer to the following text and Figure 1 for the LTC3450

power-up sequence. When input power is applied, the

boost converter initializes and charges its output towards

the final value of 5.1V. When the boost converter output

reaches approximately 90% of its final value (4.5V), an

internal 5V OK signal is asserted which allows the charge

pump doubler to begin operation toward its final goal of

10V. Approximately 1ms later, the charge pump inverter

begins operation toward its final goal of either

approximately 50% of its final value, a 4ms (nominal)

timeout period begins. At the conclusion of the 4ms

timeout period, the charge pump tripler is allowed to

begin operation, which will eventually charge V3X to 15V

(nominal).

–10V

5V

10V

15V

1ms

3450 F01

4ms

Figure 1. Output Sequencing